Premature or preterm delivery of otherwise normal babies is a problem throughout the world, in both developed and developing countries. In fact, delivery of infants before the completion of 37 weeks of gestation is the leading cause of neonatal morbidity and mortality in the United States, McCormick, N. Engl. J. Med. 312:82-90 (1985), where the incidence of such deliveries has hovered around 7-9% for many years. McCormack M., "Trends in Rates of Low Birthweight in the United States," In Berendes HW, Kessel S., Yaffe S., (eds.) Advances In The Prevention Of Low Birthweight, Washington, D.C.: National Center for Education in Maternal and Child Health, 1991:3-11. Studies have shown that preterm neonates account for more than half, and perhaps as much as 75%, of the mortality and morbidity among newborns without congenital abnormalities. Rush et al., BMJ 2:965-8 (1976). Although various intervention programs have claimed success in reducing the number of preterm deliveries, the results have been difficult to reproduce or sustain. See, e.g., Creasy et al., Obstet. Gynecol. 76:Suppl: 2S-4S (1990). Thus, as perinatal mortality and morbidity due to other causes have decreased, the relative magnitude of the problem of preterm delivery has grown, despite the significant improvement in neonatal care. Creasy, N. Engl. J. Med. 325(10):727-8 (1991).
The medical profession has acknowledged that preventing preterm labor or rupture of the membranes is far more desirable than dealing with the problem after the fact. Id. A solution to the problem has been exacerbated because preterm birth is a multifaceted problem with different causes. Most of the current approaches to the prevention of preterm birth rely in part on risk-scoring systems to identify a group of women to whom special attention can be directed. Id. These rely on such factors as obstetrical history, demographic factors, and premonitory symptoms. Main et al., Am. J. Obstet. Gynecol. 151:892-8 (1985). However, these approaches have been roundly criticized as being neither sensitive nor specific. Id. For example, application of these methods to multiple pregnancies, including bed rest in pregnancies with twins, has been shown to be of little value. MacLennan et al., Lancet 335:267-9 (1990). Current treatment regimens have also been frequently hampered by an advance stage of labor or the inability to distinguish between irrelevant contractions and true preterm labor. Lockwood et al., N. Engl. J. Med. 325(10):669-74 (1991). Thus, effective preventative measures have been essentially unavailable, largely because of the inability to predict the problem with enough certainty to warrant enrolling patient in a trial of preventative approaches. Creasy et al., supra.
A major focus of research has been to find a biochemical marker predictive of spontaneous preterm labor or premature rupture of the fetal membranes. Various candidates for biochemical markers of preterm delivery, e.g., plasma estradiol-17 beta, progesterone, and C-reactive protein, have not withstood rigorous scrutiny. Lockwood, supra. Lockwood et al. studied fetal fibronectin as a marker candidate, based upon the previous identification of this ubiquitous plasma and extracellular matrix protein in amniotic fluid and placental tissue. Matsuura et al., Proc. Natl. Acad. Sci. USA 82:6517-21 (1985). Lockwood's hypothesis was that mechanical or inflammatory-mediated damage to the fetal membrane before preterm delivery results in the release of fibronectin into the cervix and vagina. Lockwood was optimistic about the results; however, the group cautioned that further studies must be conducted to determine whether the presence of fibronectin in cervicovaginal secretions can not only identify asymptomatic patients at risk for term delivery, i.e., contractions are recognized, but also determine the time interval between a positive test and the beginning of preterm labor so that therapy could be initiated. Lockwood's conclusions were later criticized on the ground that the fetal-fibronectin test may have a clinically important false positive rate if used to diagnose ruptured membranes. Creasy et al., supra. Creasy cast further doubt in Lockwood's work by offering an alternative hypothesis for the presence of fetal fibronectin in cervicovaginal fluids which did not implicate fetal membranes.
The chorioamniotic membranes are essentially connective tissue structures. Since collagen determines the tensile strength of fibrous connective tissue, there has been considerable interest in investigating collagen biochemistry in the setting of premature fetal membrane rupture. See Romero et al., Contemp. OB/GYN, May 1993, 33-44. Some investigators have detected low collagen content in membranes that have ruptured prematurely as compared with normal membranes, and suggested that the tensile property of the amniotic membranes in the former cases must be lower than in normal membranes. See Skinner et al., Obstet. Gynecol. 57:487-9 (1981). However, other groups have found no difference in the connective tissue collagen content in prematurely ruptured fetal membranes. See Al-Zaid et al., Br. J. Obstet. Gynaecol. 87:227-9 (1980); Evaldson et al., Gynecol. Obstet. Invest. 29:92-4 (1987). There have been yet other reports of high levels of collagenolytic activity in prematurely ruptured fetal membranes and in the serum of women with preterm labor. See, e.g., Vadillo-Oretega, Obstet. Gynecol. 75:84-8 (1990). However, the precise mechanism for these biochemical changes has remained unknown. See Katsura et al., FEBS Letters 244(2):315-18 (1989); So et al., Biol. Reprod. 46:772-8 (1992). Enzymes capable of degrading collagen have been previously described as products of cultured cells derived from fetal membranes. So et al., Acta. Obst. Gynaec. (Jpn) 45(3):227-233 (1993).
The immunocytochemical detection of collagenase (MMP-2) in fetal membranes was reported in Fernandez et al., Lab. Invest. 66:572-579 (1992). The enzyme family known as matrix metalloproteinases (MMPs) has been implicated in many normal tissue remodeling processes such embryonic development, postpartum involution of the uterus, bone and growth plate remodeling, ovulation, and wound healing, as well as pathological conditions such as arthritis, tumor invasion and metastasis. See Woessner, FASEB J. 5:2145-54 (1991). While these enzymes may act on certain common substrates such as denatured collagen (gelatin) in vitro, they undoubtedly have specific natural substrates in vivo which account for their distinct roles in specific cellular processes. The enzymes described by So et al., namely MMP-1 and MMP-3, have not been correlated with structural changes in fetal membranes.
Hence, a need remains for a more specific and reliable biochemical marker with which to diagnose or predict fetal membrane rupture.